Antioxidative Mechanisms of Sulfite and Protein-Derived Thiols during Early Stages of Metal Induced Oxidative Reactions in Beer

The radical-mediated reactions occurring during the early stages of beer storage were studied by following the rate of oxygen consumption, radical formation as detected by electron spin resonance spectroscopy, and concentrations of the antioxidant compounds sulfite and thiols. Addition of either Fe(III) or Fe(II) had similar effects, indicating that a fast redox equilibrium is obtained between the two species in beer. Addition of iron in combination with hydrogen peroxide gave the most pronounced levels of oxidation due to a direct initiation of ethanol oxidation through generation of hydroxyl radicals by the Fenton reaction. The concentration of sulfite decreased more than the thiol concentration, suggesting that thiols play a secondary role as antioxidants by mainly quenching 1-hydroxyethyl radicals that are intermediates in the oxidation of ethanol. Increasing the temperature had a minor effect on the rate of oxygen consumption.